The reasons for feed-in interruptions due to grid incidents are generally not defects in the inverter. Rather, the grid connection area was frequently identified as an error source in practice.

But to what extent does the grid connection area influence the inverter?

Three main points can be mentioned here:

1. An insufficient line cross-section between the feed-in point (e.g. domestic connection box) and meter cabinet or between the meter cabinet and the inverter

2. Transition resistances on terminal and fuse elements

3. Voltage, impedance and frequency fluctuations in the public utility grid

Re 1.) Insufficient line cross-section

A sufficient line cross-section is required in order to feed the power generated by the inverter into the grid. To this end, all influencing factors must be taken into consideration when dimensioning the cables. The main such factors are the ambient temperature, the manner in which the cable is laid, the insulation’s temperature resistance, the distance from the inverter to the feed-in point, the level of grid voltage and the maximum possible current.

If the line cross-section between the inverter and feed-in point is too low, the line resistance may be too high for the power to be fed in. However, in order to feed its entire power into the grid, the inverter raises the voltage at its AC output. If the AC line is now too under-dimensioned, the voltage at the inverter output can rise above the value permitted in the standard. In this case, the inverter must automatically disconnect from the grid.

In order, in practice, to state whether the line cross-section in the present case is sufficiently dimensioned, it is helpful to calculate the voltage drop during rated operation and/or measure it.

In Sunny Design, the power loss of the selected cable dimensioning can be calculated here. SMA Solar Technology AG recommends a relative power loss with rated operation of less than 1%.

When selecting the conductor cross-sections, always comply with the national and international standards.

Re 2.) Transition resistances on terminal and fuse elements

Excessively high transition resistances can also influence the voltage rise. These can arise, for example, if, in addition to the cable ends, a cable’s insulation is accidentally trapped on terminal devices and fuse elements, screw connections on the circuit breakers are not tightened or the assembly of the AC connection point in the inverter is not performed properly. In these cases, the connection conditions from the inverter to the feed-in point must be optimized.

Re 3.) Voltage, impedance and frequency fluctuations in the public utility grid

Three features of the utility grid are the grid voltage, grid impedance and grid frequency. Their level is determined mainly by the low-voltage transformer, the cable cross-section, the cable length and the connected loads. If the permissible limit values for the grid voltage or grid frequency are overshot, the inverter must detect this in accordance with the VDEW guideline requirements and interrupt its feed-in operation.

An excessively low or excessively high voltage or frequency at the feed-in point can therefore lead to unwanted inverter shutdowns.

The following measures can help to improve the situation:

• Adjusting the voltage

The grid operator can react to the voltage rise in the grid caused by the generator by adjusting the voltage on the upstream low-voltage transformer.

• Expanding the grid

If adjusting the voltage does not improve the situation, it may be a good idea to expand the grid. A larger line cross-section can prevent impedance fluctuations and excessively high voltages at the feed-in point. Generally, the grid operator is in charge of expanding their line grid. Alternatively, the grid operator can check whether a different feed-in point is more suitable.

• Changing the grid interface parameters on the inverter

If expanding the grid is problematic or uneconomical, the grid operator can grant permission to change the voltage and frequency parameters of the grid interface on the inverter. The parameters can be changed by an approved electrically qualified person. As changes to the grid interface are nevertheless relevant to safety, a personal access code (grid guard code) is required for the change. The electrically qualified person can apply to SMA Solar Technology AG for this code. The application form can be downloaded from the SMA homepage at www.SMA.de (download area). The subsequent procedure for changing the operator parameters is described in the inverter operating and installation manual.

Note: If the voltage value is increased when the grid interface parameters are changed, this can have a negative impact on other loads in individual cases.

All work on the inverter and the cabling of the PV array must be carried out by an electrically qualified person. SMA accepts no liability for the information and solutions given in the Online Service Center. Additional information can be found in our Online Service Center’s terms and conditions of use.